In recent years, the use of cellular communication systems having mobile devices which communicate with a hardwired network, such as a local area network (LAN) or a wide area network (WAN), has become widespread. Retail stores and warehouses, for example, may use cellular communication systems with mobile data terminals to track inventory and replenish stock. The transportation industry may use such systems at large outdoor storage facilities to keep an accurate account of incoming and outgoing shipments. In manufacturing facilities, such systems are useful for tracking parts, completed products and defects. Such systems are also utilized for cellular telephone communications to allow users with wireless telephones to roam across large geographic regions while retaining telephonic access. Paging networks also may utilize cellular communication systems which enable a user carrying a pocket sized pager to be paged anywhere within a geographic region.
A typical cellular communication system includes a number of fixed base stations interconnected by a cable medium often referred to as a system backbone. Also included in many cellular communication systems are intermediate base stations which are not directly connected to the system backbone but otherwise perform many of the same functions as the fixed base stations. Intermediate base stations, often referred to as wireless base stations, increase the area within which base stations connected to the system backbone can communicate with mobile devices. Unless otherwise indicated, the term "base station" will hereinafter refer to both base stations hardwired to the system backbone and wireless base stations.
Associated with each base station is a geographic cell. Such cell is a geographic area in which a base station has sufficient signal strength to transmit data to and receive data from a mobile device such as a data terminal or telephone with an acceptable error rate. Typically, base stations will be positioned along the backbone such that the combined cell area coverage from each base station provides full coverage of a building or site.
Mobile devices such as telephones, pagers, personal digital assistants (PDAs), data terminals, etc. are designed to be carried throughout the system from cell to cell. Each mobile device is capable of communicating with the system backbone via wireless communications between the mobile device and a base station to which the mobile device is registered. As the mobile device roams from one cell to another, the mobile device will typically deregister with the base station of the previous cell and register with the base station associated with the new cell.
Cellular communication systems such as those described above often involve spread spectrum (SS) technology. An SS communication system is one in which the transmitted frequency spectrum or bandwidth is much wider than absolutely necessary. Generally, SS technology is utilized for communications in the unlicensed bands provided by the FCC for low power communication devices. These bands include the 902-928 MHZ and 2.4-2.48 GHz ranges in the U.S., although SS communication may occur in any allowable range. The FCC requires that information transmitted in these bands be spread and coded in order to allow multiple user access to these bands at the same time.
One type of SS communication system is known as a frequency hopping spread spectrum (FHSS) system. The coding scheme for a FHSS system utilizes a pseudo-random hopping sequence whereby information is sent using a sequence of carrier frequencies that change at intervals to produce a narrow band signal that "hops" around in center frequency over the available spectrum. Only transmitters and receivers hopping on the same sequence are capable of communication with one another. Thus, multiple users can share the same bandwidths without significant interference by selecting different pseudo-random hopping sequences with which to communicate.
The FCC provides rules governing the use of FHSS systems. For example, if communicating in the 2.4-2.48 GHz unlicensed band, the FCC provides that FHSS systems must have at least 75 hopping frequencies, or channels, separated by at least 25 kHz, and the average time of occupancy (or "dwell time") on any given channel must not be greater than 0.4 seconds in any 30 second period. This means that a maximum possible dwell time on any given channel is 400 milliseconds (msec), and typically will be about 100 msec.
In a FHSS system, each base station is typically required to communicate using a different hopping sequence including different channels and/or a different order of channels. Therefore, in order for a mobile device to roam from cell to cell, it must be able to "lock-on" to each new hopping sequence it encounters. In conventional systems, mobile devices typically use either an active or passive scanning mode to lock-on to a new hopping sequence associated with a new base station to which it wishes to register upon loss of communications with the base station to which it had been registered. Unfortunately, there have been several drawbacks associated with conventional active or passive scanning as will now be discussed.
FIG. 1 represents the sequence of operations involved in a typical active scanning mode. If operating in the active scanning mode, a mobile device initially selects a channel and sends out a probe packet to determine whether any base station within range is currently communicating on that channel. The mobile terminal then waits for a predetermined period of time during which a probe response packet should be received from the base station on that channel provided a base station is currently on the channel. More specifically, in step 100 the mobile device sets its transmitter and receiver to operate on a selected one of the possible hopping frequencies or channels within the system. Next, in step 102 the mobile device transmits a probe packet on the selected channel. The probe packet indicates to any base station within range and communicating on the same channel that the mobile device would like information regarding the particular hopping sequence employed by the receiving base station.
In step 104, the mobile device determines if a probe response packet has been received by the mobile device within a predetermined period of time (e.g., 6 msec) following the transmission of the probe packet. If no probe response packet is received in step 104, the mobile device selects another possible hopping channel within the system and sets its receiver and transmitter to operate on the newly selected channel as represented in step 106. Following step 106, the mobile device returns to step 102 and transmits a probe packet on the newly selected channel. Steps 102, 104 and 106 are repeated until the mobile device receives a probe response packet and is able to lock-on to a new hopping sequence. More specifically, when a probe response is received in step 104, the mobile device proceeds to step 108 in which the hopping sequence of the base station is determined based on the contents of the probe response packet. Timing information included in the probe response packet allows the mobile device to then lock-on to the hopping sequence. Typically, the amount of time the mobile device remains on any one channel while actively scanning is short compared to the amount of time a base station dwells on a given channel. Therefore, the mobile terminal can scan through each possible channel and will ultimately receive a probe response, albeit after some time delay. Depending on where a base station is in its hopping sequence and the order in which the mobile device selects different channels on which to transmit a probe packet, the mobile device may have to cycle through all of the possible hopping channels (e.g., all 75 or more channels) numerous times before hitting on the same channel that a base station within range is currently on in its hopping sequence. Thus, a time delay may exist anywhere between 0 to 10 seconds, for example, before the mobile device determines the hopping sequence of a new base station with which to register.
In a passive scanning mode, a mobile device does not send out probe packets to determine whether a base station is currently on the same channel. Rather, the base stations are configured to periodically transmit beacon packets indicating the particular hopping sequence utilized by the base station. Each mobile device simply stays on a given one of the possible hopping channels and waits to receive a beacon packet from a base station. The beacon packet provides the mobile device with hopping sequence and timing information which allows the mobile device to lock-on to the new hopping sequence.
FIG. 2 represents another passive scanning mode technique in which a mobile device periodically hop from channel to channel waiting to receive a beacon packet. For example, in step 110 the mobile device sets its receiver to operate on a selected one of the possible hopping channels. Next, in step 112 the mobile device stays on the selected channel and waits a predetermined period of time (e.g., 10 msec) to receive a beacon packet on the selected channel. In step 114 the mobile device determines if a beacon packet was received. If no, the mobile device proceeds to step 116 in which the mobile device selects another hopping channel and sets its receiver to operate on the newly selected channel. Thereafter, the mobile device returns to step 112 and again waits a predetermined time to receive a beacon packet. Steps 112, 114 and 116 are repeated until such time as a beacon packet is received as determined in step 114. At that time, the mobile device proceeds to step 118 in which the mobile device locks on to the hopping sequence of the base station transmitting the beacon packet based on the information provided in the beacon packet.
Therefore, by remaining on one channel or by sequencing through the various hopping channels waiting to receive a beacon packet, the mobile device will eventually receive a beacon packet. However, as with the active scanning mode there will be an indefinite time delay before a beacon packet is received and the mobile device is able to lock-on to the hopping sequence of another base station. Such time delay could be, for example, anywhere from zero to ten seconds.
Unfortunately, during those times that a mobile device is not registered to a base station or is otherwise attempting to register with a new base station, no communication can occur between the mobile device and devices situated on the system backbone. As a result, users often experience down time where it appears that their mobile device has locked up so as not to permit communications. This can be both frustrating to the user and detrimental to the overall system performance. Similar situations may also occur in other systems having base stations each communicating on different communication channels as produced by using different modulation types or PN codes, for example.
In view of the aforementioned shortcomings associated with conventional cellular systems, there is a strong need in the art for a system and method which help minimize the delay times associated with mobile devices locking on to new hopping sequences or other communication channels when roaming from one cell to another, or searching for a different communication channel in the same cell area. Moreover, there is a strong need in the art for improved active and passive scanning techniques which further reduce conventional delay times.